This invention pertains to the field of sensing a position of an elevator motor, and in particular, to sensing the position of the motor without using an encoder.
There exists a considerable interest in the motion control industry for AC drives that do not use any mechanical position or speed sensors. Such a mode of operation is commonly referred to as sensorless control. Since shaft sensors are expensive, delicate, and require additional cabling, removing shaft sensors from motion control configurations is likely to result in reduced system costs while improving overall reliability.
A synchronous AC motor can achieve a desired torque production only if the applied stator waveforms, typically voltages, have phase which is coordinated with the rotor position. A knowledge of the rotor position is thus essential for smooth operation of an elevator drive.
The present invention uses characteristics of a permanent magnet synchronous motor (PMSM) to determine the position of the motor. An electric motor relies on two basic principles of electromagnetism. First, whenever electric current is passed through a wire, an electromagnetic field is created around the wire with a strength proportional the amount of current, and second, whenever wire is moved through a magnetic field, electric current is generated in the wire which depends on the strength of the magnetic field, the size of the wire, and the speed and distance at which the wire moves through the magnetic field.
A PMSM motor is an AC motor that includes a stationary section called the stator, and a rotating section called the rotor. Wire windings and north-south magnetic poles in the stator are arranged so that an AC current introduced into the windings causes a magnetic field that rotates around the stator. This rotating magnetic field then induces a current in the rotor. The induced rotor current produces a magnetic field that chases the rotating stator field, thus causing the rotor, which is attached to a shaft, to turn. The speed of rotation of the magnetic field is called the synchronous speed of the motor. The rotor electromagnet tries to catch the rotating stator magnetic field, but as it approaches alignment with the stator, the rotating magnetic field no longer intersects the rotor conductors, so the induced current decreases and the rotor field decreases. The rotor then slows down and xe2x80x9cslipsxe2x80x9d below the speed of the stator""s rotating magnetic field. As the rotor slows, more conductors are intersected by the rotating magnetic field, causing the rotor current and field to increase again, resuming the chase of the stator field. The synchronous motor in a steady state tends to run at a nearly constant speed depending on the frequency of the applied AC voltage. The AC motor torque is a function of the amplitude and phase of the applied AC voltage.
Attempts have been made to quickly and reliably determine the rotor position of a synchronous motor. T. Aihara, A. Toba, T. Yanase, A. Mashimo, and K. Endo, in xe2x80x9cSensorless Torque Control of Salient-Pole Synchronous Motor at Zero-Speed Operationxe2x80x9d, IEEE Trans. Power Electronics, 14(10), 1999, pp. 202-208, disclose a position and speed sensorless control using the counter emf of a PMSM. A voltage signal is injected into the d-axis and observe the response in the q-current. Processing is done with a fast Fourier transform (FFT).
M. J. Corley and R. D. Lorenz, in xe2x80x9cRotor Position and Velocity Estimation for a PMSM at Standstill and High Speedsxe2x80x9d, IEEE IAS Annual Meeting, 1996, pp. 36-41, disclose adding probing voltage signals to both axes (but dominantly in the d-axis, which is labeled as q-axis in the paper); the q-current is later used for position determination. The algorithm shows a steady-state offset.
U.S. Pat. Nos. 5,585,709 and 5,565,752 (Jansen et al.), both entitled xe2x80x9cMethod and Apparatus for Transducerless Position and Velocity Estimation in Drives for AC Machinesxe2x80x9d disclose injecting a high frequency signal onto the fundamental drive frequency in the stator windings of the motor.
J. M. Kim, S. J. Kang, and S. K. Sul, in xe2x80x9cVector Control of Interior PMSM Without a Shaft Sensorxe2x80x9d, Applied Power Electronics Conference, 1997. pp. 743-748, disclose a control method wherein the switching pattern is modified by injecting a probing current signal into the q-axis, and voltage responses are processed to evaluate the positional error. This means that the injected frequency has to be well within the bandwidth of the current regulator.
U.S. Pat. No. 5,886,498 (Sul et al.) entitled xe2x80x9cSensorless Field Orientation Control Method of an Induction Machine by High Frequency Signal Injectionxe2x80x9d discloses injecting a fluctuating high frequency signal at a reference frame rotating synchronously to the fundamental stator frequency.
U.S. Pat. No. 5,608,300 (Kawabata et al.) entitled xe2x80x9cElectrical Angle Detecting Apparatus and Driving System of Synchronous Motor Using the Samexe2x80x9d and U.S. Pat. No. 5,952,810 (Yamada et al.) entitled xe2x80x9cMotor Control Apparatus and Method for Controlling Motorxe2x80x9d consider cases when both step and sinusoidal signals are injected in the motor. In the case of sinusoidal injection, a band-pass filter is used to extract the important part of the response. These disclosures concentrate on initial position detection of the rotor.
U.S. Pat. No. 5,903,128 (Sakakibara et al.) entitled xe2x80x9cSensorless Control System and Method of Permanent Magnet Synchronous Motorxe2x80x9d discloses modifying the supply voltage so that the position can be inferred from the system response. The modification, however, is quite simple (one or two pulses), so it is likely to result in limited performance.
Briefly stated, a permanent magnet synchronous motor is controlled using d-axis current and q-axis current along with respective d-current and q-current feedback loops. In one embodiment of the invention which is described in detail below, the speed and position of the motor are determined by injecting a first signal into the d-axis current and observing the response in the q-current feedback loop. Part of the q-current feedback signal is demodulated with a second signal which is 90 degrees out of phase from the first signal. The demodulated signal is preferably sent through a low pass filter before being received by an observer controller. The observer controller outputs a position estimate which is used to modify the d-current feedback signal and a speed estimate which is used to modify the q-axis current.
According to an embodiment of the invention, an apparatus for determining speed and position of a permanent magnet synchronous motor includes d-axis current means for providing d-axis current to the motor; the d-axis current means including a d-current feedback loop; q-axis current means for providing q-axis current to the motor; the q-axis current means including a q-current feedback loop; means for injecting a first signal into the d-axis current means; position estimating means for measuring current feedback responsive to the injected first signal to determine an estimated position of the motor; and speed estimating means for measuring current feedback responsive to the injected first signal to determine an estimated speed of the motor.
According to an embodiment of the invention, a method for determining speed and position of a permanent magnet synchronous motor includes the steps of (a) providing d-axis current to the motor, including providing a d-current feedback loop; (b) providing q-axis current to the motor, including providing a q-current feedback loop; (c) injecting a first signal into the d-axis current outside of the d-current feedback loop; (d) measuring current feedback responsive to the injected first signal to determine an estimated position of the motor; and (f) measuring current feedback responsive to the injected first signal to determine an estimated speed of the motor.